1. Field of the Invention
The present invention relates, in general, to a bit data reproduction apparatus for an optical disc playback system, such as a compact disc player or a digital versatile disc player and, more particularly, to a frequency detection method for an optical disc bit data reproduction apparatus, which rapidly detects the frequency of input data, required for data bit reproduction, from radio frequency analog data that are input to a reception unit of an optical disc playback system having various read speeds.
2. Description of the Related Art
Bits of digital information, that is, 0 and 1, are recorded on the surface of an optical disc, such as a Compact Disc (CD) or Digital Versatile Disc (DVD). Bit information indicated by 0 and bit information indicated by 1 differ from each other in reflexibility of light. An optical disc pickup generates a Radio Frequency (RF) analog waveform shown in FIG. 1 using amounts of laser light, reflected from an optical disc, that correspond to bit values of 0 or 1. A reception unit of an optical disc playback system reproduces digital bit information recorded on a disc by sampling the RF analog waveform. In this case, the frequency and phase information of data input to the reception unit is required, and the reception unit detects the frequency and phase using the input RF waveform. In order to accurately reproduce digital data in an optical disc playback system having various data input frequencies, it is important to rapidly and accurately detect frequency and phase.
For a method of detecting frequency information from a CD or DVD, methods using a sync code are frequently used. In a CD and DVD, a specific sync code for identifying predefined data bit units exists. If a time period corresponding to one bit is 1T, a CD and a DVD are used by identifying a signal having a length that is equal to two successive lengths of 11T and a signal having a length of 14T, as sync codes, respectively. A frequently used conventional method to detect the frequency of input data is to compare the length of a sampled sync code with the length of a sync code, obtained when sampling is performed at a correct frequency, and to determine whether a current sampling frequency is high or low. That is, a sync code has a greater code length than typical data codes of a CD or DVD which have limited code lengths, such as a minimum length of 3T and a maximum length of 11T. Therefore, the sync code is first detected and the length of a sampled sync code is then stored. In the case of a DVD, if the length of the sampled sync code is greater than 14T, a current sampling frequency is higher than the frequency of input data. In contrast, if the length of the sampled sync code is less than 14T, a current sampling frequency is lower than the frequency of input data. Therefore, such a frequency error is corrected, thus detecting an accurate input frequency.
However, this method is disadvantageous in that, if defects, such as scratches or fingerprints, exist on the surface of an optical disc, or if data recorded on the optical disc are not clear, it is possible to miscalculate the length of a sync code and detect an erroneous current sampling frequency, so that the frequency of input data may not be correctly detected. Therefore, it is necessary to detect an accurate sync code and sample the sync code. In this case, there is problem in that, in order to detect an accurate sync code when an input frequency deviates considerably from a sampling frequency, a certain period of time is required, thus increasing the time required to detect an input frequency.
Further, Korean Pat. Laid-Open Publication No. 1998-020547 discloses a method of decreasing a sampling frequency when a signal having a length less than 3T, such as 1T or 2T, is detected, and increasing a sampling frequency when a signal having a length greater than 11T is detected. This patent discloses a method of correcting a frequency when data having a length exceeding limit lengths are detected, using the characteristics of optical disc bit data limiting a minimum successive length and a maximum successive length of bit data to 3T and 11T, respectively. This method is problematic in that an input frequency cannot be rapidly and stably detected, for example, the time during which a frequency detector adjusts a sampling frequency is not uniform according to the status and type of an optical disc and the frequency of input data, and frequency may not be satisfactorily adjusted according to a gain value and may not be correctly detected when scratches exist on the surface of a disc.